FIGS. 11(a)-11(b) are schematic diagrams of a laser diode (LD) with a waveguide lens according to a prior art. In the figure, reference numeral 1 designates an n-InP substrate, numeral 3 designates an n-InP cladding layer formed on the n-InP substrate 1, numeral 2 designates a waveguide 0.1 .mu.m thick comprising an InGaAs/InGaAsP multi quantum well layer formed on the n-InP cladding layer 3, and numeral 4 designates a p-InP cladding layer formed on the n-InP cladding layer 3 so as to bury the waveguide 2. The waveguide 2 comprises a portion 2a of laser active layer having a width of about 1 .mu.m in a laser region LD, and comprises a waveguide portion 2b with a tapered width at a waveguide portion (lens portion) L, and the width of the tip is about 0.3 .mu.m. A spot size of the tip of the taper shaped waveguide portion 2b is about several .mu.m.
FIGS. 12(a)-12(b) are schematic diagrams of a laser diode (LD) with a waveguide lens according to another prior art. In these figures, the same reference numerals as in FIGS. 11(a)-11(b) designate the same or corresponding parts. Reference numeral 22 designates a waveguide, numeral 22a designates a laser active layer portion in a laser region LD as in the FIGS. 11, and numeral 22b designates a waveguide portion 22b with a tapered width in a lens portion L. The thickness of the tip of the waveguide portion 22b is about one fifth of that of the waveguide 22a of the LD portion. Here, the LD with a waveguide lens is an LD having an integrated waveguide lens for efficiently transmitting laser beam from the LD to an optical fiber.
Conventionally, a spot size (about 1 .mu.m) of laser beam guiding through a waveguide of an LD is quite smaller than a core size (about 10 .mu.m) of an optical fiber, so that it is difficult to couple directly an optical fiber with an LD, and efficient coupling of light is not feasible. In order to avoid the above-described difficulty, a waveguide lens widens a spot size of laser beam from LD before coupling into an optical fiber. In fact, the waveguide lens is narrowed as the waveguide portion 2b in the vicinity of a laser beam emitting facet FA as shown in FIG. 11(a), or is thinned in a tapered shape as the waveguide portion 22b as shown in FIGS. 12(a)-12(b), so that the optical confinement is reduced, whereby a waveguide lens widens the spot size of laser beam to about 5 .mu.m. However, the LD with a waveguide lens according to a prior art still has difficulties in the fabricating method, which will be described below.
Initially, in order to obtain a sufficient spot size with using the structure of FIG. 11(a), the narrowest portion of the waveguide 2 is required to be about 0.3 .mu.m However, performing a high-accuracy photolithograpy and etching in this size are very difficult with using the LSI technique or the like.
On the other hand, there are two methods for fabricating the structure of FIGS. 12 as follows,
&lt;1&gt; The LD portion is made to be thicker than the lens portion using layer thickness increasing effect by selective growth of a semiconductor layer using a mask. PA1 &lt;2&gt; The waveguide at a lens portion is made to be a taper shape and to be more thin than that of LD using a specific etching.
Where the thickness of the tip of the lens portion of the waveguide is about one fifth of that of the LD portion as described above.
A description is given of the method &lt;1&gt;.
FIG. 13(a) shows a selective growth mask pattern for a crystal growth of a semiconductor layer by MOCVD, and FIG. 13(b) shows a profile of the thickness along c-c' of the semiconductor layer grown using the mask. In the region where the laser active layer sandwiched by the selective growth masks 31 of LD portion LD is formed, excessive raw material species are supplied from the surface of mask 31 by vapor phase diffusion, so that the growth rate is increased and the layer becomes thick as compared with that in the region which has no mask. Since the quantity of the vapor phase diffusion is varied dependent on raw material species, a composition of the growing layer is varied in the region sandwiched by the masks as compared with the region of the lens portion L which has no mask.
In the case where InGaAsP is grown, InGaAsP having a large composition ratio of In is grown in the region sandwiched by the masks of the LD portion LD because the quantity of the vapor phase diffusion of In is larger than that of Ga, where In and Ga are device of III group. The lattice constant of InGaAsP is large in proportion as a quantity of In is large in the ratio of In and Ga, so that a difference in the lattice constant between the growing crystal and the substrate is large, whereby crystal defects occur, as a result, the quality of the crystal is deteriorated.
As another problem of this method &lt;1&gt;, because there is five times difference between the thick part and the thin part of-the growing layer on the substrate (it is required to provide such difference as described above), a difference in level is occurred at the boundary portion between the two parts, and this difference in level may be an obstacle in the subsequent process of producing device.
In the above-described method &lt;1&gt;, it is necessary to make the thickness of the LD portion five times as thick as the other portion. Assuming that InGaAs/InGaAsP multi quantum well waveguide layer is grown, the composition is varied (the ratio of In to Ga, and the ratio of As to P are varied) in the selective growth region sandwiched by the masks, and it is difficult to suppress the thickness of the LD portion with accuracy. As a result, difficulties in the quality of the crystal and suppressing the wavelength with accuracy or the like are occurred by lattice strain due to the change of composition.
On the other hand, two methods are given as the method &lt;2&gt; in following subsections.
As the one method, as shown in FIG. 14(a), initially, a waveguide 22 is formed by alternately laminating InGaAaP layers 19a.about.19e and the InP etching stopper layers 14a.about.14e. Next, an etching mask 20 is formed at the required position on the uppermost surface layer 19a, and using this mask, the InGaAsP layer 19a is etched by etchant, i.e., nitric acid or the like, until the etching is stopped by an InP etching stopper-layer. Thereafter, the exposed portion of the InP etching stopper layer 14a beyond the InPGaAsP layer 19a is etched by etchant which etches InP and does not etch InGaAsP, i.e., hydrochloric acid, whereby the two upper layers shown in FIG. 14(b) are formed. Respective layers 19b, 14b, 19c, 14c, 19d, 14d, 19e, and 14e are formed in the above-described process, resulting in the structure of FIG. 14(b).
As the other method, as shown in FIG. 15(a), an oxide film 15 having a tapered portion is formed on the waveguide 22, and as shown in FIG. 15(b), the waveguide 22 is etched by ion milling 15b through the oxide film 15, whereby a tapered waveguide portion 22b having a tapered thickness is formed. However, these methods have particular and complicated processes, and the surface of the waveguide layer is directly etched and regrowth of crystal is performed thereon. Therefore, there is a fear of deteriorating the reliability of the device.